I. Field of the Invention
The present invention relates generally to mobile communication networks such as, for example, cellular telephone systems. More specifically, the present invention relates to a novel and improved system and method for communicating information, in mobile cellular or satellite mobile telephone systems, using spread spectrum communication signals.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several methods for facilitating communications in systems accommodating a large number of users. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention.
In the above-referenced U.S. Pat. No. 4,901,307, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
The CDMA techniques as disclosed in U.S. Pat. No. 4,901,307 contemplated the use of relatively long high speed pseudonoise (PN) sequences with each user channel being assigned a different PN sequence. The cross-correlation between different PN sequences and the autocorrelation of a PN sequence for all time shifts other than zero both have a zero average value which allows the different user signals to be discriminated upon reception.
However, because such PN signals are not orthogonal mutual interference noise is created therebetween. This interference noise arises despite the fact that the cross-correlations of the PN signals average to zero, since for a short time interval such as an information bit time the cross-correlation follows a binomial distribution. As such, the user signals interfere with each other much the same as if they were wide bandwidth Gaussian noise at the same power spectral density. Accordingly, mutual interference noise arising from non-orthogonal user signals tends to limit achievable system capacity.
In U.S. Pat. No. 5,103,459, issued 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", also assigned to the assignee of the present invention, and which is herein incorporated by reference, there is disclosed a novel and improved method and system for constructing PN sequences that provide orthogonality between the users so that mutual interference will be reduced. Such a reduction in mutual interference allows for higher system capacity and better link performance. Since with orthogonal PN codes the cross-correlation is zero over a predetermined time interval, no mutual interference arises provided that the code time frames are aligned with each other.
In the system described in the just mentioned patent, a preferred waveform design implemented involves a direct sequence PN spread spectrum carrier. The chip rate of the PN carrier was chosen to be 1.2288 MHz in the preferred embodiment. One consideration involved in the choice of chip rate is that it be exactly divisible by the baseband data rates to be used in the system. It is also desirable for the divisor to be a power of two. In the preferred embodiment, the baseband data rate is 9600 bits per second, leading to a choice of 1.2288 MHz, 128 times 9600 for the PN chip rate.
In communications between a cellular base station and a mobile unit over the cell-to-mobile link, the code sequences used for spreading the spectrum are constructed from two different types or sequences, each with different properties to provide different functions. There is an outer code that is shared by all signals in a cell or sector that is used to discriminate between multipath signals. The outer code is also used to discriminate between signals transmitted by different cells or sectors to the mobile units. There is also an inner code that is used to discriminate between user signals transmitted within a single sector or cell.
In certain instances it may be desired that the voice channels within a cell operate at variable data rates. The intent in using a variable data rate is to lower the data rate when there is no voice activity thereby reducing interference generated by the particular voice channel to other users. In this regard U.S. Pat. No. 5,414,796, issued May 9, 1995, entitled "VARIABLE RATE VOCODER", also assigned to the assignee of the present invention, discloses a vocoder producing data at four different data rates based on voice activity on a 20 msec frame basis. Exemplary data rates are 9.6 kbps, 4.8 kbps, 2.4 kbps and 1.2 kbps.
In addition to providing variable rate voice channels, it is also desired to allow various types of user channels (e.g., voice, facsimile, or high-speed data) to operate at different data rates. In such variable data rate systems it may be known in advance that the maximum data rate over a certain type of user channel will never be required to exceed some rate, e.g., 4.8 kbps, lower than a nominal system rate of 9.6 kbps. In principle, twice as many user channels should be capable of being provided at a data rate of 4.8 kbps relative to the number which could be made available at a rate of 9.6 kbps. However, if the set of orthogonal user codes were initially generated assuming a fixed transmission rate of 9.6 kbps, the available number of codes may be insufficient to support such a two-fold increase in the number of users.
Such a variable data rate system could also include user channels, such as those dedicated to high-speed data transmission, requiring data rates of double the nominal rate of 9.6 kbps, i.e., rates of 19.2 kbps. One obvious method for providing operation at 19.2 kbps would simply be to assign two Walsh channel codes to such users and divide the transmitted bits between the two channels. However, this would complicate system design by requiring that demodulators be provided for each of the two assigned Walsh codes.
It is therefore an object of the present invention to provide a Walsh coding technique enabling the orthogonal coexistence of high and low data rate channels in a spread spectrum communication system.
It is a further object of the invention that such a coding technique maximize the number of available channels by efficiently assigning codes of varying length to high and low data rate channels.